Functional characterization and quantitative expression analysis of two GnRH-related peptide receptors in the mosquito, Aedes aegypti

Functional analysis of adipokinetic hormone signaling in reproductive diapause of Coccinella septempunctata

BACKGROUND

The ladybeetle, Coccinella septempunctata, an important predator, is widely used to control aphids, whiteflies, mites, thrips, and lepidopteran pests. Diapause control technology is key to extending C. septempunctata shelf-life and commercialization. Lipid accumulation is a major feature of reproductive diapause, but the function of AKH signaling as a regulator of lipid mobilization in reproductive diapause remains unclear. This study aimed to identify and characterize AKH and AKHR genes, and clarify their functions in reproductive diapause.

RESULTS

The relative expression levels of CsAKH and CsAKHR were the highest in the head and fat body, respectively, and were significantly decreased under diapause conditions, both in developmental stages and tissues (head, midgut, fat body, and ovary). Furthermore, CsAKH and CsAKHR expression was increased significantly after juvenile hormone (JH) injection, but CsMet silencing significantly inhibited CsAKH and CsAKHR expression, whereas CsMet knockdown blocked the induction effect of JH. CsAKH and CsAKHR knockdown significantly reduced water content, increased lipid storage, and promoted the expression of genes related to lipid synthesis, but significantly blocked ovarian development, and induced forkhead box O (FOXO) gene expression in C. septempunctata under reproduction conditions. By contrast, injection of AKH peptide significantly inhibited FOXO expression, reduced lipid storage, and increased water content in C. septempunctata under diapause conditions.

CONCLUSION

These results indicate that CsAKH and CsAKHR are involved in the regulation of lipid accumulation and ovarian development during diapause in C. septempunctata, and provide a promising target for manipulating C. septempunctata diapause. © 2024 Society of Chemical Industry.

Lipids in Insect Reproduction: Where, How, and Why

Modern insects have inhabited the earth for hundreds of millions of years, and part of their successful adaptation lies in their many reproductive strategies. Insect reproduction is linked to a high metabolic rate that provides viable eggs in a relatively short time. In this context, an accurate interplay between the endocrine system and the nutrients synthetized and metabolized is essential to produce healthy offspring. Lipids guarantee the metabolic energy needed for egg formation and represent the main energy source consumed during embryogenesis. Lipids availability is tightly regulated by a complex network of endocrine signals primarily controlled by the central nervous system (CNS) and associated endocrine glands, the corpora allata (CA) and corpora cardiaca (CC). This endocrine axis provides hormones and neuropeptides that significatively affect tissues closely involved in successful reproduction: the fat body, which is the metabolic center supplying the lipid resources and energy demanded in egg formation, and the ovaries, where the developing oocytes recruit lipids that will be used for optimal embryogenesis. The post-genomic era and the availability of modern experimental approaches have advanced our understanding of many processes involved in lipid homeostasis; therefore, it is crucial to integrate the findings of recent years into the knowledge already acquired in the last decades. The present chapter is devoted to reviewing major recent contributions made in elucidating the impact of the CNS/CA/CC-fat body-ovary axis on lipid metabolism in the context of insect reproduction, highlighting areas of fruitful research.

Contribution of the transcription factor SfGATAe to Bt Cry toxin resistance in Spodoptera frugiperda through reduction of ABCC2 expression

Insect resistance evolution poses a significant threat to the advantages of biopesticides and transgenic crops utilizing insecticidal Cry-toxins from Bacillus thuringiensis (Bt). However, there is limited research on the relationship between transcriptional regulation of specific toxin receptors in lepidopteran insects and their resistance to Bt toxins. Here, we report the positive regulatory role of the SfGATAe transcription factor on the expression of the ABCC2 gene in Spodoptera frugiperda. DNA regions in the SfABCC2 promoter that are vital for regulation by SfGATAe, utilizing DAP-seq technology and promoter deletion mapping. Through yeast one-hybrid assays, DNA pull-down experiments, and site-directed mutagenesis, we confirmed that the transcription factor SfGATAe regulates the core control site PBS2 in the ABCC2 target gene. Tissue-specific expression analysis has revealed that SfGATAe is involved in the regulation and expression of midgut cells in the fall armyworm. Silencing SfGATAe in fall armyworm larvae resulted in reduced expression of SfABCC2 and decreased sensitivity to Cry1Ac toxin. Overall, this study elucidated the regulatory mechanism of the transcription factor SfGATAe on the expression of the toxin receptor gene SfABCC2 and this transcriptional control mechanism impacts the resistance of the fall armyworm to Bt toxins.

Authentication of a lophotrochozoan adipokinetic hormone receptor in a Gastropod, Aplysia californica

Gonadotropin-releasing hormone (GnRH) superfamily comprises multiple families of signaling peptides in both protostomes and deuterostomes. Among this superfamily, vertebrate GnRH stimulates reproduction, but other GnRH superfamily members elicit diverse pleiotropic effects. Within the GnRH superfamily members, adipokinetic hormone (AKH) and its receptor are well described in ecdysozoans but understudied in other lineages. To fill this knowledge gap, we deorphanized a putative receptor for a lophotrochozoan AKH in a gastropod mollusk, Aplysia californica, and named it Aplca-AKHR. Phylogenetic analysis revealed an orthologous relationship of Aplca-AKHR with ecdysozoan AKHRs and other putative lophotrochozoan AKHRs. Aplca-AKHR bound specifically to the previously identified Aplca-AKH with high affinity and activated the inositol phosphate pathway. Aplca-AKHR was expressed widely among central and peripheral tissues, but most prominently in several central ganglia and the heart. The expression of Aplca-AKHR was downregulated by a hyposaline challenge, consistent with a role in volume and fluid regulation previously described for its ligand, Aplca-AKH. In summary, this is the first pairing of a lophotrochozoan AKH with its cognate receptor. Expression data further support diverse central and peripheral roles, including volume and fluid control, of this ligand/receptor pair.

A corazonin G protein-coupled receptor gene in the tick Ixodes scapularis yields two splice variants, each coding for a specific corazonin receptor

We have identified a corazonin G protein-coupled receptor (GPCR) gene in the tick Ixodes scapularis, which likely plays a central role in the physiology and behavior of this ectoparasite. This receptor gene is unusually large (1.133 Mb) and yields two corazonin (CRZ) receptor splice variants, where nearly half of the coding regions are exchanged: CRZ-Ra (containing exon 2, exon 3, and exon 4 of the gene) and CRZ-Rb (containing exon 1, exon 3, and exon 4 of the gene). CRZ-Ra codes for a GPCR with a canonical DRF sequence at the border of the third transmembrane helix and the second intracellular loop. The positively-charged R residue from the DRF sequence is important for coupling of G proteins after activation of a GPCR. CRZ-Rb, in contrast, codes for a GPCR with an unusual DQL sequence at this position, still retaining a negatively-charged D residue, but lacking a positively-charged R residue, suggesting different G protein coupling. Another difference between the two splice variants is that exon 2 from CRZ-Ra codes for an N-terminal signal sequence. Normally, GPCRs do not have N-terminal signal sequences, although a few mammalian GPCRs have. In the tick CRZ-Ra, the signal sequence probably assists with inserting the receptor correctly into the RER membrane. We stably transfected Chinese Hamster Ovary cells with each of the two splice variants and carried out bioluminescence bioassays that also included the use of the human promiscuous G protein G₁₆. CRZ-Ra turned out to be selective for I. scapularis corazonin (EC₅₀ = 10^-8 M) and could not be activated by related neuropeptides like adipokinetic hormone (AKH) and AKH/corazonin-related peptide (ACP). Similarly, also CRZ-Rb could only be activated by corazonin, although about 4-fold higher concentrations were needed to activate it (EC₅₀ = 4 x 10^-8 M). The genomic organization of the tick corazonin GPCR gene is similar to that of the insect AKH and ACP receptor genes. This similar genomic organization can also be found in the human gonadotropin-releasing hormone (GnRH) receptor gene, confirming previous conclusions that the corazonin, AKH, and ACP receptor genes are the true arthropod orthologues of the human GnRH receptor gene.

Multiple endocrine factors regulate nutrient mobilization and storage in Aedes aegypti during a gonadotrophic cycle

Anautogenous mosquitoes must blood feed on a vertebrate host to produce eggs. Each gonadotrophic cycle is subdivided into a sugar-feeding previtellogenic phase that produces primary follicles and a blood meal-activated vitellogenic phase in which large numbers of eggs synchronously mature and are laid. Multiple endocrine factors including juvenile hormone (JH), insulin-like peptides (ILPs), ovary ecdysteroidogenic hormone (OEH), and 20-hydroxyecdysone (20E) coordinate each gonadotrophic cycle. Egg formation also requires nutrients from feeding that are stored in the fat body. Regulation of egg formation is best understood in Aedes aegypti but the role different endocrine factors play in regulating nutrient mobilization and storage remains unclear. In this study, we report that adult female Ae. aegypti maintained triacylglycerol (TAG) stores during the previtellogenic phase of the first gonadotrophic cycle while glycogen stores declined. In contrast, TAG and glycogen stores were rapidly mobilized during the vitellogenic phase and then replenishment. Several genes encoding enzymes with functions in TAG and glycogen metabolism were differentially expressed in the fat body, which suggested regulation was mediated in part at the transcriptional level. Gain of function assays indicated that stored nutrients were primarily mobilized by adipokinetic hormone (AKH) while juvenoids and OEH regulated replenishment. ILP3 further showed evidence of negatively regulating certain lipolytic enzymes. Loss of function assays indicated AKH depends on the AKH receptor (AKHR) for function. Altogether, our results indicate that the opposing activities of different hormones regulate nutrient stores during a gonadotrophic cycle in Ae. aegypti.

Functional insight and cell-specific expression of the adipokinetic hormone/corazonin-related peptide in the human disease vector mosquito, Aedes aegypti

The adipokinetic hormone/corazonin-related peptide (ACP) is an insect neuropeptide structurally intermediate between corazonin (CRZ) and adipokinetic hormone (AKH). Unlike the AKH and CRZ signaling systems that are widely known for their roles in the mobilization of energy substrates and stress responses, respectively, the main role of ACP and its receptor (ACPR) remains unclear in most arthropods. The current study aimed to localize the distribution of ACP in the nervous system and provide insight into its physiological roles in the disease vector mosquito, Aedes aegypti. Immunohistochemical analysis and fluorescence in situ hybridization localized the ACP peptide and transcript within a number of cells in the central nervous system, including two pairs of laterally positioned neurons in the protocerebrum of the brain and a few ventrally localized neurons within the pro- and mesothoracic regions of the fused thoracic ganglia. Further, extensive ACP-immunoreactive axonal projections with prominent blebs and varicosities were observed traversing the abdominal ganglia. Given the prominent enrichment of ACPR expression within the abdominal ganglia of adult A. aegypti mosquitoes as determined previously, the current results indicate that ACP may function as a neurotransmitter and/or neuromodulator facilitating communication between the brain and posterior regions of the nervous system. In an effort to elucidate a functional role for ACP signaling, biochemical measurement of energy substrates in female mosquitoes revealed a reduction in abdominal fat body in response to ACP that matched the actions of AKH, but interestingly, a corresponding hypertrehalosaemic effect was only found in response to AKH since ACP did not influence circulating carbohydrate levels. Comparatively, both ACP and AKH led to a significant increase in haemolymph carbohydrate levels in male mosquitoes while both peptides had no influence on their glycogen stores. Neither ACP nor AKH influenced circulating or stored lipid levels in both male and female mosquitoes. Collectively, these results reveal ACP signaling in mosquitoes may have complex sex-specific actions, and future research should aim to expand knowledge on the role of this understudied neuropeptide.

Corazonin signaling modulates the synthetic activity of male accessory gland in Grapholita molesta

Although neuropeptide corazonin (Crz) has been identified in numerous insect species, the research about its function in regulation of reproduction is still in its infancy. Herein, we characterized the Crz (GmolCrz) and its receptor (GmolCrzR) to investigate their reproductive function in Grapholita molesta. Both molecular docking result and cell-based receptor activity assay showed that GmolCrz could interact with GmolCrzR. Additionally, spatial expression patterns of GmolCrz and GmolCrzR in males were evaluated. Knockdown of GmolCrz or GmolCrzR significantly lengthened copulation duration and decreased fertility in males. In these males, we found that the production of sperm was normal, while the content of accessory gland proteins (Acps) in the accessory gland (AG) was strongly diminished. Furthermore, knockdown of GmolCrz or GmolCrzR in males had no effect on sperm and Acps transfer to females. RNA-seq and gene expression analyses further confirmed that genes involved in serine-type endopeptidase activity were significantly downregulated in the AG upon GmolCrzR knockdown. Finally, sperm activation assays demonstrated that this process was disrupted in the spermatophore of females mated with GmolCrz or GmolCrzR knockdown males, which may cause the decreased fertility in males. Our findings provide new insights into the functions of Crz signaling in a Lepidopteran insect.

Identification of single domain antibodies with insect cytotoxicity using phage-display antibody library screening and Plutella xylostella ATP-binding cassette transporter subfamily C member 2 (ABCC2) -based insect cell expression system

It is extremely imminent to study a new strategy to manage agricultural pest like Plutella xylostella (P. xylostella) which is currently resistant to most of pesticides, including three domain-Cry toxins from Bacillus thuringiensis (Bt). In this study, we reported a phage displayed single domain antibody screening from human domain antibody (DAb) library targeted on Spodoptera frugiperda 9 (Sf9) cells expressed Cry1Ac toxin receptor, ATP-dependent binding cassette transporter C2 in P. xylostella (PxABCC2). After three rounds of panning, three cytotoxic antibodies (1D2, 2B7, 3C4) were obtained from thirty-eight antibodies and displayed high binding ability towards PxABCC2-expressed Sf9 cells. Through homology modeling and molecular docking, the interaction mode indicated that the most cytotoxic 1D2 of the three antibodies presented the lowest binding free energy required and had the most hydrogen bond formed with PxABCC2 in molecular docking analysis. Functional assay of key regions in 1D2 via Alanine replacement indicated that complementarity-determining region (CDR) 3 played a crucial role in antibody exerts binding activity and cytotoxicity. This study provides the first trial for discovering of potential cytotoxic antibodies from the human antibody library via specific receptor-expressed insect cell system biopanning.

The Peptide Hormone CNMa Influences Egg Production in the Mosquito Aedes aegypti

Mosquito reproduction is regulated by a suite of hormones, many acting through membrane-bound receptor proteins. The Aedes aegypti G protein-coupled receptors AAEL024199 (AeCNMaR-1a) and AAEL018316 (AeCNMaR-1b) were identified as orthologs of the Drosophila melanogaster CNMa receptor (DmCNMaR). The receptor was duplicated early in the evolution of insects, and subsequently in Culicidae, into what we refer to as CNMaR-1a and CNMaR-1b. AeCNMaR-1a is only detected in male mosquito antennae while AeCNMaR-1b is expressed at high levels in mosquito ovaries. Using a heterologous cell assay, we determined that AeCNMa activates AeCNMaR-1a with a ~10-fold lower concentration than it does AeCNMaR-1b, though both receptors displayed half maximal effective concentrations of AeCNMa in the low nanomolar range. Finally, we show that injections of AeCNMa into blood-fed mated female Ae. aegypti resulted in fewer eggs laid.

Identification of novel-vector control target proteins of Aedes sp.: A Systems Network Biology Approach

Aedes is an important vector for various viruses that cause dengue, chikungunya and zika, which affect human health globally. Due to regular outbreaks of these diseases worldwide, there is a need to identify essential vector proteins that are critical for the survival of the vector, which may be targeted to control the spread of vector-borne disease (VBD). In silico computational methods involving comparative proteomics, analysis of orthologous proteins common amongst members of Aedes genus and protein-protein interaction (PPI) pathway were used to identify essential proteins that could act as novel therapeutic candidates. Twenty-three conserved proteins between A. aegypti and A. albopictus were identified from a BLASTP search with an e-value threshold of 0.005, and their PPI networks were constructed in the STRING database. The merged network was analyzed using various Cytoscape plugins viz. ClusterONE, Cytohubba and MCODE. Thirty-one hub proteins were identified from the system's network biology analysis, and detailed data and literature mining were carried out. Twelve novel vector-control target proteins of A. aegypti, having no human homologs, were determined in the present study that can effectively act as potential therapeutic candidates for drug design and vaccine development.

The Intrinsic Nutrient Sensing Adipokinetic Hormone Producing Cells Function in Modulation of Metabolism, Activity, and Stress

All organisms confront the challenges of maintaining metabolic homeostasis in light of both variabilities in nutrient supplies and energetic costs of different physiologies and behaviors. While all cells are nutrient sensitive, only relative few cells within Metazoans are nutrient sensing cells. Nutrient sensing cells organize systemic behavioral and physiological responses to changing metabolic states. One group of cells present in the arthropods, is the adipokinetic hormone producing cells (APCs). APCs possess intrinsic nutrient sensors and receive contextual information regarding metabolic state through other endocrine connections. APCs express receptors for different hormones which modulate APC physiology and the secretion of the adipokinetic hormone (AKH). APCs are functionally similar to alpha cells in the mammalian pancreas and display a similar physiological organization. AKH release results in both hypertrehalosemia and hyperlipidemia through high affinity binding to the AKH receptor (AKHR). Another hallmark of AKH signaling is heightened locomotor activity, which accompanies starvation and is thought to enhance foraging. In this review, we discuss mechanisms of nutrient sensing and modulation of AKH release. Additionally, we compare the organization of AKH/AKHR signaling in different taxa. Lastly, we consider the signals that APCs integrate as well as recent experimental results that have expanded the functional repertoire of AKH signaling, further establishing this as both a metabolic and stress hormone.

G-Protein Coupled Receptors (GPCRs): Signaling Pathways, Characterization, and Functions in Insect Physiology and Toxicology

G-protein-coupled receptors (GPCRs) are known to play central roles in the physiology of many organisms. Members of this seven α-helical transmembrane protein family transduce the extracellular signals and regulate intracellular second messengers through coupling to heterotrimeric G-proteins, adenylate cyclase, cAMPs, and protein kinases. As a result of the critical function of GPCRs in cell physiology and biochemistry, they not only play important roles in cell biology and the medicines used to treat a wide range of human diseases but also in insects’ physiological functions. Recent studies have revealed the expression and function of GPCRs in insecticide resistance, improving our understanding of the molecular complexes governing the development of insecticide resistance. This article focuses on the review of G-protein coupled receptor (GPCR) signaling pathways in insect physiology, including insects’ reproduction, growth and development, stress responses, feeding, behaviors, and other physiological processes. Hormones and polypeptides that are involved in insect GPCR regulatory pathways are reviewed. The review also gives a brief introduction of GPCR pathways in organisms in general. At the end of the review, it provides the recent studies on the function of GPCRs in the development of insecticide resistance, focusing in particular on our current knowledge of the expression and function of GPCRs and their downstream regulation pathways and their roles in insecticide resistance and the regulation of resistance P450 gene expression. The latest insights into the exciting technological advances and new techniques for gene expression and functional characterization of the GPCRs in insects are provided.

GnRH-Related Neurohormones in the Fruit Fly Drosophila melanogaster

Genomic and phylogenetic analyses of various invertebrate phyla revealed the existence of genes that are evolutionarily related to the vertebrate’s decapeptide gonadotropin-releasing hormone (GnRH) and the GnRH receptor genes. Upon the characterization of these gene products, encoding peptides and putative receptors, GnRH-related peptides and their G-protein coupled receptors have been identified. These include the adipokinetic hormone (AKH) and corazonin (CRZ) in insects and their cognate receptors that pair to form bioactive signaling systems, which network with additional neurotransmitters/hormones (e.g., octopamine and ecdysone). Multiple studies in the past 30 years have identified many aspects of the biology of these peptides that are similar in size to GnRH and function as neurohormones. This review briefly describes the main activities of these two neurohormones and their receptors in the fruit fly Drosophila melanogaster. The similarities and differences between Drosophila AKH/CRZ and mammalian GnRH signaling systems are discussed. Of note, while GnRH has a key role in reproduction, AKH and CRZ show pleiotropic activities in the adult fly, primarily in metabolism and stress responses. From a protein evolution standpoint, the GnRH/AKH/CRZ family nicely demonstrates the developmental process of neuropeptide signaling systems emerging from a putative common ancestor and leading to divergent activities in distal phyla.

Production, composition, and mode of action of the painful defensive venom produced by a limacodid caterpillar, Doratifera vulnerans

Venoms have evolved independently several times in Lepidoptera. Limacodidae is a family with worldwide distribution, many of which are venomous in the larval stage, but the composition and mode of action of their venom is unknown. Here, we use imaging technologies, transcriptomics, proteomics, and functional assays to provide a holistic picture of the venom system of a limacodid caterpillar, Doratifera vulnerans Contrary to dogma that defensive venoms are simple in composition, D. vulnerans produces a complex venom containing 151 proteinaceous toxins spanning 59 families, most of which are peptides <10 kDa. Three of the most abundant families of venom peptides (vulnericins) are 1) analogs of the adipokinetic hormone/corazonin-related neuropeptide, some of which are picomolar agonists of the endogenous insect receptor; 2) linear cationic peptides derived from cecropin, an insect innate immune peptide that kills bacteria and parasites by disrupting cell membranes; and 3) disulfide-rich knottins similar to those that dominate spider venoms. Using venom fractionation and a suite of synthetic venom peptides, we demonstrate that the cecropin-like peptides are responsible for the dominant pain effect observed in mammalian in vitro and in vivo nociception assays and therefore are likely to cause pain after natural envenomations by D. vulnerans Our data reveal convergent molecular evolution between limacodids, hymenopterans, and arachnids and demonstrate that lepidopteran venoms are an untapped source of novel bioactive peptides.

CAPA neuropeptides and their receptor form an anti-diuretic hormone signaling system in the human disease vector, Aedes aegypti

Insect CAPA neuropeptides are homologs of mammalian neuromedin U and are known to influence ion and water balance by regulating the activity of the Malpighian 'renal' tubules (MTs). Several diuretic hormones are known to increase primary fluid and ion secretion by insect MTs and, in adult female mosquitoes, a calcitonin-related peptide (DH31) called mosquito natriuretic peptide, increases sodium secretion to compensate for the excess salt load acquired during blood-feeding. An endogenous mosquito anti-diuretic hormone was recently described, having potent inhibitory activity against select diuretic hormones, including DH31. Herein, we functionally deorphanized, both in vitro and in vivo, a mosquito anti-diuretic hormone receptor (AedaeADHr) with expression analysis indicating highest enrichment in the MTs where it is localized within principal cells. Characterization using a heterologous in vitro system demonstrated the receptor was highly sensitive to mosquito CAPA neuropeptides while in vivo, AedaeADHr knockdown abolished CAPA-induced anti-diuretic control of DH31-stimulated MTs. CAPA neuropeptides are produced within a pair of neurosecretory cells in each of the abdominal ganglia, whose axonal projections innervate the abdominal neurohaemal organs, where these neurohormones are released into circulation. Lastly, pharmacological inhibition of nitric oxide synthase (NOS) and protein kinase G (PKG) signaling eliminated anti-diuretic activity of CAPA, highlighting the role of the second messenger cGMP and NOS/PKG in this anti-diuretic signaling pathway.

Physiological and Environmental Factors Affecting the Composition of the Ejaculate in Mosquitoes and Other Insects

In addition to transferring sperm, male mosquitoes deliver several proteins, hormones and other factors to females in their seminal fluid that inhibit remating, alter host-seeking behaviors and stimulate oviposition. Recently, bioinformatics, transcriptomics and proteomics have been used to characterize the genes transcribed in male reproductive tissues and the individual proteins that are delivered to females. Thanks to these foundational studies, we now understand the complexity of the ejaculate in several mosquito species. Building on this work, researchers have begun to identify the functions of various proteins and hormones in the male ejaculate, and how they mediate their effects on female mosquitoes. Here, we present an overview of these studies, followed by a discussion of an under-studied aspect of male reproductive physiology: the effects of biotic and abiotic factors on the composition of the ejaculate. We argue that future research in this area would improve our understanding of male reproductive biology from a physiological and ecological perspective, and that researchers may be able to leverage this information to study key components of the ejaculate. Furthermore, this work has the potential to improve mosquito control by allowing us to account for relevant factors when implementing vector control strategies involving male reproductive biology.

Five Neuropeptide Ligands Meet One Receptor: How Does This Tally? A Structure-Activity Relationship Study Using Adipokinetic Bioassays With the Sphingid Moth, Hippotion eson

Adipokinetic hormones (AKHs) play a major role in mobilizing stored energy metabolites during energetic demand in insects. We showed previously (i) the sphingid moth Hippotion eson synthesizes the highest number of AKHs ever recorded, viz. five, in its corpus cardiacum: two octa- (Hipes-AKH-I and II), two nona- (Hipes-AKH-III and Manse-AKH), and one decapeptide (Manse-AKH-II), which are all active in lipid mobilization (1). (ii) Lacol-AKH from a noctuid moth showed maximal AKH activity in H. eson despite sequence differences and analogs based on Lacol-AKH with modifications at positions 2, 3, 8, or at the termini, as well as C-terminally shortened analogs had reduced or no activity (2). Here we report on N-terminally shortened and modified analogs of the lead peptide, as well as single amino acid substitutions at positions 1, 4, 5, 6, and 7 by an alanine residue. Ala¹ and Glu¹ instead of pGlu are not tolerated well to bind to the H. eson AKH receptor, whereas Gln¹ has high activity, suggesting it is endogenously cyclized. Replacing residue 5 or 7 with Ala did not alter activity much, in contrast with changes at position 4 or 6. Similarly, eliminating pGlu¹, Leu², or Thr³ from Lacol-AKH severely interfered with biological activity. This indicates that there is no core peptide sequence that can elicit the adipokinetic effect and that the overall conformation of the active peptide is required for a physiological response. AKHs achieve a biological action through binding to a receptor located on fat body cells. To date, one AKH receptor has been identified in any given insect species; we infer the same for H. eson. We aligned lepidopteran AKH receptor sequences and note that these are very similar. The results of our study is, therefore, also applicable to ligand-receptor interaction of other lepidopteran species. This information is important for the consideration of peptide mimetics to combat lepidopteran pest insects.

Insight Into Mosquito GnRH-Related Neuropeptide Receptor Specificity Revealed Through Analysis of Naturally Occurring and Synthetic Analogs of This Neuropeptide Family

Adipokinetic hormone (AKH), corazonin (CRZ), and the AKH/CRZ-related peptide (ACP) are neuropeptides considered homologous to the vertebrate gonadotropin-releasing hormone (GnRH). All three Aedes aegypti GnRH-related neuropeptide receptors have been characterized and functionally deorphanized. Individually they exhibit high specificity for their native ligands, prompting us to investigate the contribution of ligand structures in conferring receptor specificity for two of these receptors. Here, we designed a series of analogs based on the native ACP sequence and screened them using a heterologous system to identify critical residues required for ACP receptor (ACPR) activation. Analogs lacking the carboxy-terminal amidation, replacing aromatics, as well as truncated analogs were either completely inactive or had very low activities on ACPR. The polar threonine (position 3) and the blocked amino-terminal pyroglutamate are also critical, whereas ACP analogs with alanine substitutions at position 2 (valine), 5 (serine), 6 (arginine), and 7 (aspartate) were less detrimental including the substitution of charged residues. Replacing asparagine (position 9) with an alanine resulted in a 5-fold more active analog. A naturally-occurring ACP analog, with a conserved substitution in position two, was well tolerated yet displayed significantly reduced activity compared to the native mosquito ACP peptide. Chain length contributes to ligand selectivity in this system, since the endogenous octapeptide Aedae-AKH does not activate the ACPR whereas AKH decapeptides show low albeit significant activity. Similarly, we utilized this in vitro heterologous assay approach against an A. aegypti AKH receptor (AKHR-IA) testing carefully selected naturally-occurring AKH analogs from other insects to determine how substitutions of specific residues in the AKH ligand influence AKHR-IA activation. AKH analogs having single substitutions compared to Aedae-AKH revealed position 7 (either serine or asparagine) was well tolerated or had slightly improved activation whereas changes to position 6 (proline) compromised receptor activation by nearly 10-fold. Substitution of position 3 (threonine) or analogs with combinations of substitutions were quite detrimental with a significant decrease in AKHR-IA activation. Collectively, these results advance our understanding of how two GnRH-related systems in A. aegypti sharing the most recent evolutionary origin sustain independence of function and signaling despite their relatively high degree of ligand and receptor homology.

Genomics, GPCRs and new targets for the control of insect pests and vectors

The pressing need for new pest control products with novel modes of action has spawned interest in small molecules and peptides targeting arthropod GPCRs. Genome sequence data and tools for reverse genetics have enabled the prediction and characterization of GPCRs from many invertebrates. We review recent work to identify, characterize and de-orphanize arthropod GPCRs, with a focus on studies that reveal exciting new functional roles for these receptors, including the regulation of metabolic resistance. We explore the potential for insecticides targeting Class A biogenic amine-binding and peptide-binding receptors, and consider the innovation required to generate pest-selective leads for development, within the context of new PCR-targeting products to control arthropod vectors of disease.

Lipid metabolism in insect disease vectors

More than a third of the world population is at constant risk of contracting some insect-transmitted disease, such as Dengue fever, Zika virus disease, malaria, Chagas' disease, African trypanosomiasis, and others. Independent of the life cycle of the pathogen causing the disease, the insect vector hematophagous habit is a common and crucial trait for the transmission of all these diseases. This lifestyle is unique, as hematophagous insects feed on blood, a diet that is rich in protein but relatively poor in lipids and carbohydrates, in huge amounts and low frequency. Another unique feature of these insects is that blood meal triggers essential metabolic processes, as molting and oogenesis and, in this way, regulates the expression of various genes that are involved in these events. In this paper, we review current knowledge of the physiology and biochemistry of lipid metabolism in insect disease vectors, comparing with classical models whenever possible. We address lipid digestion and absorption, hemolymphatic transport, and lipid storage by the fat body and ovary. In this context, both de novo fatty acid and triacylglycerol synthesis are discussed, including the related fatty acid activation process and the intracellular lipid binding proteins. As lipids are stored in order to be mobilized later on, e.g. for flight activity or survivorship, lipolysis and β-oxidation are also considered. All these events need to be finely regulated, and the role of hormones in this control is summarized. Finally, we also review information about infection, when vector insect physiology is affected, and there is a crosstalk between its immune system and lipid metabolism. There is not abundant information about lipid metabolism in vector insects, and significant current gaps in the field are indicated, as well as questions to be answered in the future.

Corazonin Signaling Is Required in the Male for Sperm Transfer in the Oriental Fruit Fly Bactrocera dorsalis

Corazonin (Crz) is a widely distributed neuropeptide (or neurohormone) in insects with diverse physiological functions. The present study aimed to reveal the functions of Crz and its receptor (CrzR) in the regulation of sexual behavior and fertility in male Bactrocera dorsalis. Tissue-specific expression analyses showed that the BdCrz transcript was most abundant in the central nervous system (CNS), and the BdCrzR transcript was most abundant in both the fat body and CNS. Immunochemical localization confirmed that three pairs of Crz-immunoreactive neurons are located in the dorsolateral protocerebrum region of male adult brain. Importantly, RNAi-mediated Crz knockdown lengthened mating duration in males, and knockdown of Crz or CrzR strongly decreased male fertility in the following 3 days, while the courtship behavior and mating efficiency were not affected. The reduced number of sperm in the reproductive organs of mated females indicated that Crz knockdown in males reduced sperm transfer. The findings of this study indicate that Crz contributes to the reproductive physiology of the oriental fruit fly B. dorsalis by regulating sperm transfer in male adults.